1. Field of the Invention
The present invention relates to semiconductor devices and, in particular, relates to DRAM devices having improved charge storage capabilities.
2. Description of the Related Art
The Dynamic Random Access Memory (DRAM) device is an important component of electronic systems requiring digital storage capabilities, such as digital computers, personal data assistants (PDA), and the like. The typical DRAM provides an array of memory cells disposed in a high density configuration. For example, a single DRAM device having a minimum feature dimension of 0.15 micron can include an array of millions of directly addressable memory cells that are each capable of storing a single bit of digital data.
FIG. 1 schematically illustrates a typical memory cell 20 comprising a single capacitor 21 and a field effect transistor (FET) 22. The capacitor 21 stores a quantity of charge which is indicative of the state of the memory cell 20. The FET 22 acts as a switch that provides a conducting path to the capacitor 21 when the cell 20 is being addressed. The FET 22 also isolates the capacitor 21 when the memory cell 20 is in a quiescent state, i.e., not being addressed, so that the capacitor 20 can store charge for extended periods of time.
As shown in FIG. 1, the FET 22 is formed within a substrate 21 and the capacitor 21 formed adjacent the substrate 23 in an ILD layer 31. The substrate 23 includes first and second doped regions 24, 25 and a channel 26 extending therebetween. For example, the first doped 24 region can be configured to be a source input of the FET 22 and the second doped region 25 can be configured to be a drain input of the FET 22. The FET 22 further comprises a gate electrode 27 disposed over the channel 26 which extends between the source 24 and drain 25 of the FET 22 and is insulated from the substrate 23 by a thin insulating layer 28. Furthermore, the capacitor 21 comprises first and second plates 29, 30 separated by an insulating layer 31, wherein the first plate 29 is coupled to the source of the FET 22 and the second plate 30 is coupled to a fixed reference voltage.
As is well known in the art, when the voltage applied at the gate 27 is greater than a threshold value, the channel 26 becomes a conducting path that allows charge to readily flow between the source 24 and drain 25. Thus, in response to a write signal arriving at the drain via a digit line 32, charge is able to flow through the channel 26 to the first plate 29 of the capacitor 21, thereby providing the capacitor 21 with a quantity of charge which is indicative of the input write signal. Likewise, during a read cycle, an output read signal can be developed at the drain 25 of the FET 22 which is indicative of the charge stored in the capacitor 21. Furthermore, during quiescent periods when the memory cell 20 is not being addressed, the gate voltage is reduced below the threshold value so as to inhibit conduction across the channel 26 and, thereby, help preserve the charge of the capacitor 21.
A common problem with memory cells of the prior art is that the capacitor is unable to store charge indefinitely. For example, even if the gate voltage is maintained below the threshold value, a subthreshold leakage current usually flows through the channel 26 which discharges the capacitor during the quiescent period. To accommodate such discharging, typical DRAM devices also include refresh circuitry which periodically monitors the state of each memory cell and repeatedly recharges individual capacitors having a detected residual charge. However, because the DRAM is inaccessible during the refresh cycle, such refreshing reduces the rate at which data can be read from or written to the device. Thus, there is a need to reduce the subthreshold leakage currents so as to extend the time between refresh cycles and, therefore increase the throughput of the device.
One method that can be used to reduce subthreshold leakage currents involves disposing complementary dopant atoms in the channel region of each FET of the memory array. For example, if the source and drain of the FET include pentavalent dopant atoms, such as Phosphorous, the complementary dopant atoms would comprise trivalent dopant atoms, such as Boron. The presence of the complementary dopant atoms within each channel region increases the corresponding threshold gate voltage and, as a result, also increases the resistance of the channel region during quiescent periods.
To ensure that each FET of the memory array is configured with substantially uniform operating characteristics, i.e. uniform gate threshold values, it is important for each channel region to be doped in a substantially identical manner. For example, a first channel region having complementary dopant atoms disposed throughout the first channel region will provide a substantially different gate threshold value than that of a second channel region having a reduced concentration of complementary dopant atoms near its edges. Thus, because of the difficulty of precisely embedding complementary dopant atoms only within the relatively narrow confines of the channel of the typical memory array using conventional masking techniques, the industry has adopted the practice of doping the complementary atoms using a blanket implant process. In other words, no attempt is made to prevent some of the complementary atoms from becoming implanted within regions of the substrate immediately adjacent the channel regions.
The blanket implant process of the prior art comprises exposing a relatively large portion of the substrate corresponding to the entire memory array to a source of energetic complimentary dopant atoms. As a result, complementary dopant atoms 33 are embedded not only within the channel 26 but also in the doped source and drain regions 24, 25 that surround the channel 26 as shown in FIG. 1 such that the concentration of complementary dopant atoms is substantially uniform across the source 24, channel 26 and drain 25. Unfortunately, the increased concentration of complementary dopant atoms 33 in the source region 24 effectively forms a diode junction with adjacent regions of the substrate 23. Consequently, the blanket implant process of the prior art produces a relatively large junction leakage current that flows from the source 24 directly into the surrounding substrate 23. Thus, the reduction in the subthreshold leakage current is gained at the expense of the increase in the junction leakage current which is undesirable since it contributes to the discharging of the capacitor 21.
From the foregoing, therefore, it will be appreciated that there is a need for a memory array having reduced discharge rates and methods for providing the same. In particular, to provide reduced subthreshold leakage currents, there is a need for the channel regions of the substrate corresponding to the array of memory cells to include complementary dopant atoms disposed so as to increase the threshold gate voltage. Furthermore, to reduce junction leakage currents, there is a need for the complementary dopant atoms to be substantially absent from actively doped regions of the substrate that are coupled to the charge storing capacitors. Finally, there is a need for the complementary dopant atoms to be disposed in such a way that the channel regions of the memory arrays becomes conductive at substantially uniform threshold voltages.
The aforementioned needs are satisfied by the memory array and method of manufacturing the same of the present invention. In one aspect, the present invention comprises a memory array of a DRAM device that comprises a plurality of memory cells for storing information, wherein the memory cells are arranged into a plurality of cell pairs such that each pair includes first and second cells each comprising a storage element for storing charge and a valve element for controlling the flow of charge to and from the corresponding storage element. The valve elements of each cell pair are disposed adjacent each other and the charge storage element of the cell pair are outwardly disposed form the valve element of the cell pair. The array further comprises a substrate comprising a plurality of valve regions and a plurality of storage regions wherein the first and second valve elements of each cell pair overlap a corresponding valve region of the plurality of valve regions. The first and second storage elements of each cell pair are electrically coupled to respective first and second storage regions of the plurality of storage regions and the valve regions of the substrate include a first concentration of complementary dopant atoms. The storage regions of the substrate include a reduced second concentration of complementary dopant atoms, thereby reducing the rate at which charge escapes from the charge storage element during quiescent periods.
In another aspect, the present invention comprises a method of reducing leakage currents in a DRAM device. The method comprises selectively implanting complementary dopant atoms within a first region of the substrate and then forming a plurality of field effect transistors each comprising first and second active nodes and a channel extending therebetween. The first active node is disposed in the substrate so that a substantial portion of the first active node is disposed outside of the first region so as to reduce a junction leakage current and the channel and second active node are disposed within the first region so as to reduce subthreshold leakage current.